In the related art, as a filler of a semiconductor sealing resin, crystalline silica powder is frequently used.
Thermal conductivity of silica powder is poor. Therefore, it is difficult for the silica powder to counteract a trend in which an amount of heating of a semiconductor increases as the semiconductor becomes increasingly highly integrated and the electric power of the semiconductor is increased.
In recent years, power devices using wide band-gap semiconductors (SiC, GaN, and the like) have been frequently used, and the devices are heated at the time of operation. Therefore, in order to smoothly dissipate heat and to keep the semiconductor stably operating, an insulating filler having thermal conductivity better than those of silica is required.
For such an insulating filler, the use of alumina (thermal conductivity: 20 to 35 W/mK), boron nitride (30 to 50 W/mK), magnesium oxide (45 to 60 W/mK), aluminum nitride (180 to 270 W/mK), and the like have been examined. Among these, in view of stability, costs, and the like, alumina is the most frequently used currently.
However, even in a case where alumina is used, alumina is still insufficient for being used in a power device.
Although the use of boron nitride instead of alumina has been examined, boron nitride has crystalline anisotropy, and accordingly, a thermal conductivity of boron nitride in a direction perpendicular to a plane is low while an in-plane thermal conductivity thereof is high. Therefore, in a case where boron nitride is kneaded with a resin, thermal conductivity thereof is insufficient. Furthermore, the costs of boron nitride also tend to be high.
As materials having a thermal conductivity higher than that of alumina or boron nitride, there are magnesium oxide and aluminum nitride. Particularly, although the price of magnesium oxide is practically equivalent to that of alumina, a thermal conductivity thereof is not less than two times a thermal conductivity of alumina.
However, both of magnesium oxide and aluminum nitride are insufficiently resistant to water. Accordingly, magnesium oxide and aluminum nitride easily react with moisture in the air or in a resin, and thermal conductivity thereof markedly deteriorates after reaction.
Therefore, as a method for improving water resistance, a method of coating surfaces of particles is frequently used. For example, a method of coating particle surfaces with glass (PbO—B2O3—SiO2) (PTL 1), a method of coating particle surfaces with a silane coupling agent (PTL 2), a method of coating particle surfaces with poorly soluble phosphate (PTL 3) or sulfate (PTL 4) or with carbon (PTL 5), and the like have been suggested.
However, all of the above methods require a high-temperature process for coating and a water resistance-improving effect thereof is insufficient.
Owing to its high thermal conductivity, a metal material such as copper is drawing attention as a high-efficiency heat radiation material. However, because such a metal is easily oxidized, thermal conductivity thereof easily deteriorates. Furthermore, metals cannot be used as an insulating thermal conductive filler because they are conductive materials. Therefore, a surface coating technique which can impart insulating properties while maintaining a high thermal conductivity of a metal is anticipated.
For example, PTL 6 suggests a method of coating surfaces of metal particles by carbonizing a thermoplastic resin such as polyvinyl chloride by heating. However, in this method, the resin needs to be fired at a high temperature (1,000° C. or higher) in an inert gas atmosphere, and the costs are high, and the productivity is poor. In addition, in the above method, an oxidation-reduction reaction between a resin and a metal oxide caused at a high temperature is used. Accordingly, depending on the type of metal, a large amount of impurities such as metal carbides is mixed in.